Half—saturation constants (K) for NO_3 uptake were measured on eight clones, belonging to three species, of marine phytoplankton. The clones isolated from oceanic, low—nutrient areas had K values of below 0.75 (mu)M NO_3, while clones of the same species taken from estuarine regions had higher K values (over 1.5 (mu)M NO_3). Other isolated from shelf areas had intermediate (1.2 (mu)M NO_3) or low (0.1 (mu)M NO_3) K values. These data suggest that physiological races of marine phytoplankton are adapted to high or low nutrient levels.